Process for fabrication of heat exchangers



c. s. SIMPELAAR PROCESS FOR FABRICATION OR HEAT ExOHANGERs FiledApril 1,1949 A O N/ f/// /1 /1 l/1/ /l /1 ,l ,l ,l /1 1/ I l 1/ l Nov. 9, 1954United States Patent() PROCESS FOR FABRICATION F HEAT EXCHANGERS ClydeS. Simpelaar, Racine, Wis., assignor to Modine Manufacturing Company,Racine, Wis., a corporation of Wisconsin Application April 1, 1949,Serial No. 84,885

9 Claims. (Cl. 29-470) The invention relates generally to heatexchangers, and the like, and more particularly to a novel process forfabricating the same. The invention is particularly adapted for use infabricating heat exchangers s uch as those illustrated in my co-pendingapplication Serial No. 780,251, iiled on October 16, 1947, now U. S.Patent No. 2,606,007, granted August 5, 1952, in which the var1- ouselements comprising the heat exchanger are bonded into a single integralunit.

The invention has among its objects the utilization of a novel processwhereby a perfect bond between the various elements of the structure,having high corrosive resistance, is achieved, with the elimination ofall internal stresses in the various bonded joints.

Another object of the invention is the utilization of such a process bymeans of which such as exchanger may be eiiciently bonded in a minimumof time and utilizing a minimum amount of heat to perform the bondingoperation, thereby also reducing the cost of manufacture.

A further object of the invention is the utilization of a novel processwhich may be practiced with the use of relatively simple and inexpensivemachinery, at the same time permitting the eicient fabrication ofrelatively large, heavy heat exchangers, with the elimination ofdestructive annealing of the respective elements and resultant weakeningof the fabricated exchanger.

Many other objects and advantages of the construction herein shown anddescribed will be obvious to those skilled in the art from thedisclosure herein given.

To this end my invention consists in the novel combination of stepsherein shown and described, and more particularly pointed out in theclaims.

In the drawings, wherein like reference characters represent like orcorresponding parts:

Fig. l is a sectional view of one form of a machine which could beemployed in conjunction with the process herein described;

Fig. 2 is a sectional 2-2 of Fig. l; and

Fig. 3 is an enlarged sectional view of a portion of a heat exchanger,illustrated in Figs. 1 and 2, of the type to which the present processis particularly applicable.

As clearly illustrated in my copending application, hereinbeforereferred to, such exchangers comprise a plurality of slabs of finnedpasses, each adapted to conduct a respective fluid, the slabs beingstacked one upon the other with separator sheets therebetween, eachrespective pass being sealed adjacent its edges by a suitable bordermember. Assuming the exchanger is adapted to handle two fluids,alternate passes would be operatively connected to one another bysuitable inlets and outlets, with the intermediate passes therebetweenlikewise operatively connected to one another by suitable inlets andoutlets, examples of which are illustrated in my co-pending application.Fig. 3 of the drawing illustrates, in section, a small portion of suchtype of exchanger having alternate slabs, indicated generally by thenumeral 1, of interlocked fins 2, and an intermediate slab 3 ofinterlocked fins 4 positioned between the slabs 1, the respective slabsbeing separated by solid sheets 5. A cover sheet 6 extends across theoutermost slab, and the peripheral edges of the slabs are sealed byborder members 7 for the slabs 1, and member 8 for the slab 3. In theconstruction illustrated, each of the iins 2 and 4 are provided withside walls 9 terminating in offset anges 11, the latter being adapted tooverlie a portion of the View taken approximately on line 2,693,636Patented Nov. 9, 1954 side wall 9 of the next adjacent iin, as clearlyillustrated.

The tins, border members, separator sheets, as well as the particularinlet and outlet ttings are preferably bonded into a single integralstructure in a single honding operation, thereby eliminating numerousseparate pieces, as well as separate attaching elements, such as screws,rivets, etc.

Heat exchangers of the type described nd particular application incommercial gas reduction plants as, for example, in connection with theproduction of commercial oxygen and nitrogen. Consequently, such type ofunits may be of considerable size and weight, having over two squarefeet of cross-sectional area, and in length of over ten feet, wherebythe weight of the finished structure may be a ton or more.

lt will be appreciated that in working with exchangers of this size, andwhich must be constructed for high pressure and very low temperatureoperation, that eiiiciency of extreme importance.

of the bonding operation is Similarly, numerous complications arise asthe size of the unit is increased, as a result of which attempts tofabricate large size units by conventional methods, such as, forexample, the employment of baking ovens or furnaces, or the use of hotgases have proved unsuccessful due to the non-uniformity of heatingwhereby distortion is produced as a result of the temperaturediiferentials thus created in the structure, and excessive annealing andpartial exhaustion of flux activity as a result of the necessary longheating period required. Likewise, as the application of pressure on theexchanger during the bonding operation is particularly desirable, theuse of an oven or the passage of hot gases through the unit isimpractical from a commercial standpoint, apart from the higher costresulting from the low production rate of the equipment employed.

In fabricating heat exchangers of this type, a number of features havebeen found to be of importance for the successful fabrication ofrelatively large units, and the present invention is directed to aprocess by means of which these features may be achieved. One of suchfeatures to which the invention is directed is the achieving of maximumspeed in heating the exchanger to bonding temperatures, therebypreventing excessive annealing of the heat exchanger parts, particularlythe extremely light thin members, as well as reducing flux deteriorationprior to the melting of the bonding material and reducing cost from thestandpoint of time required in the bonding operation.

A second desirable feature is the ability to cool the unit ouicklv afterthe bonding material has melted, without disturbing the assembly, alsoassisting in preventing annealing and givingy a quick solidication ofthe bonding material, resulting in a finer grained` better bondstructure, as well as resulting in a cost reduction from a timestandpoint. It is believed apparent that the use of a process reuuiringan oven structure. or the like, would be highly impractical toaccomplish either rapid heating or rapid cooling, particularly coolingwithout removal of the unit from such an oven.

Another important factor in the fabrication of large units of the typedescribed is the elimination of temperature differentials within planesparallel to the separator sheets of the exchanger. It will be apparentthat. in large size units, considerable expansion and contraction of theelements is involved, particularly in the separator sheets, and if theheating of the assembled unit is such that the temperature adjacent theperiphery of any of the separator sheets is greater than that of thecenter portions. stresses are introduced in the particular sheets sothat during the heating of the unit the expansion would be greatestadjacent the periphery of the sheets, while in cooling the process wouldbe reversed and the periphery of the sheets would initially contract,resulting in bulging or waviness in the center of the sheets. Obviouslythis distortion taking place during the solidication of the bondingmaterial would also result in a disturbance of the bond during thecritical instant of solidication and may even result in actualseparation of the bond. Likewise, if not actually destroyed, the bondwould be weakened due to its solidication under conditions of changingstress, whereby internal stresses between various portions of theexchanger would represent initialstress loading of the joints with aresultant' reduction 1n the ultimate strength of the finished structure.h

Another important feature in the successful fabrication of such units isthe use of controlled pressure to maintain parts of the structure inintimate contact during the compacting operation as the bondingvmaterial'melts, and as the' amount of campaetion in a large un1t may be3/8 inch or more, it is particularly desirable to successively melt thebonding material in portions of the structure rather than. for thebonrding material to simultaneously melt throughout the structure,whereby the compacting movement is in the form of numerous slightmovements rather than one large movement. It will also be` apparent thatinl a heat exchanger unit such as that illustrated, failure to haveuniform bonding temperatures in planes extending parallel to theseparator sheets w1ll result in failure of the sheets to uniformlycompact into position as non-melted center sections` would result D1ntemporary high spots. Such non-uniformity likewise 1ncreases the poortemperature distribution and tends to give greater access of air to theperipheral bonding surfaces, destroying the capillary space, increasingthe exhaustion of ux activity, and tending to cause segregation of uxand bonding material.

In view of the above, it is believed apparent that commonly used methodsof bonding are wholly inadequat'e in the fabrication of exchangers ofthis type. The structure illustrated in Figs. l and 2 of the drawing ismerely illustrative of one type of structure which could be employed inconnection with the practice of the process herein described, and isdisclosed primarily to facilitate the explanation of such process.

Referring to the drawing, 11 indicates generally a heat exchangerstructure constructed, for example, as illustrated in Fig. 3, in whichcase the separator and outer sheets would extend horizontally. Theexchanger structure 1'1 is supported between a lower metallic plate 12and an upper metallic plate 13, the size ofthe two plates being atleasty co-extensive with the longitudinal and transverse dimensions ofthe exchanger 11. The lower plate 12 is carried by a base member,indicated generally by the numeral 14, a layer of suitable heatinsulating material I being interposed therebetween, and, in likemanner, the upper plate 13 is carried by an upper supporting structureI6, a layer of heat insulating material 17 being interposed therebetweenwith the plate 13 secured to the member 16 by any suitable means.Positioned adjacent the longitudinal side walls of the heat exchanger 11are respective layers of heat insulating material 18 which aremaintained in position by vertical walls 19, the latter, in the deviceillustrated, being suitably secured to the piece 14 by bolts 21, or thelike. The ends of the exchanger 11 likewise are covered by respectivelayers 22 of insulating material suitably mounted on plate members 23,the latter, in the device illustrated, being hinged to the uppersupporting member 16, as indicated at 24. The lower member 14 may besupported by any suitable means, and the upper member 16 is supported bysuitable means operative to apply pressure through the plate 13 to theheat exchanger 11, whereby such pressure is uniformly distributed overthe entire surface of the exchanger. Both plates 12 and 13 are adaptedto be heated by suitable units as, for example, electrical heatingelements 25 which, in the device illustrated, areA suitably embedded inthe respective plates. Suitable cooling means is alsoprovided which, inthe construction illustrated, comprises aplurality of transverselyextending passages 26 adapted to be operatively connected to a supplyline of suitable coolant as, for example, wet steam. Thus pressure maybe exerted on the exchanger 11 by the plates 12 and 13 and, at the sametime, heat or relative cold may be applied to the outermost sheets ofthe exchanger by conduction from the plates 12 and 13. It will beapparent that the plates 12 and 13, together with their respectivesupporting structures, may be constructed whereby the upper plate 13 islmovable relative to the lower plate 1n a vertical direction, at the sametime maintaining the two plates parallel to one another, with the sidewalls 19 and end walls 23 constructed to permit such movement.Regardless of the particular type of structure employed to achieve thedesired results, it will be ynoted such a structure would require arelatively small mass to b e heated or cooled, whereby the same could bedone rapidly with a minimum amount of heat or coolant. Likewise,substantially complete inrmobility` of the exchanger structure may beachieved during both the heating and cooling operation, apart frominherent dimensional changes in the exchanger structure.

In fabricating heat exchangers in accordance with the present invention,the bonding material may be incorporated in the exchanger in one ofseveralways at the time of, or prior to, assembly, for example, eitherby coating desired surfaces of elements to be bonded with bondingmaterial, or by laying sheets of bonding material between surfaces ofelements tol be bonded, or a combination in the assembly of both,whereby the assembled exchanger includes the kdesired amount of bondingmaterial. Where a coatingv of bonding material is employed, only one ofany pair of surfaces tobe. bonded together would normally be coated withbonding material. Bonding ux may be applied by any suitable means to anyexposed or uncoated surfaces to be bonded, either prior to assembly orvsubsequent thereto, depending upon the nature. of the structureinvolved, the flux employed preferablyv having a highly volatile vehicleof low latent heat, and

including a metallic salt. The utiliaztion of such a typev flux in theprocess reduces, the amount of heat` required to supply the latent heatof evaporation of they flux, as well as. eliminating the tendency ofgasV pocketing in the, joints, resulting with ux vehicles; of lessvolatility. The; use of a metallic salt, such as a. tin salt, serves adual function in the process in that it not only insures completewetting of all exposed or uncoated bondingA surfaces with bondingmaterial, but results in the addition of tin to the bonding material,with improved corrosion resistance to moisture in the final bond. Theamount of tin; so added to the bonding material would appear to be inthe neighborhood of about 1A of one percent, and results in a bond ofimproved qualities and of particular advantage' in exchangers of thetype involved.

Following the application of flux, the exchanger is then positioned in abonding device, such as that herein described, pressurey is appliedtothe outer sheets, and the heat conduction. plates are rapidly broughtup to bonding temperatures. I have found that very satisfactory resultsmay be obtained by the use of pressures of from 5 to 8 pounds per squareinch of exterior surface. The bond-ing material employed is preferablyone having a relatively very narrow liquidus-solidus range, wherebyrapid solidilication of the joints is obtained, permitting rapidcoolingy of the assembly without the introduction of stresses in thebond during the soliditication period, which may take place prior to theapplication of artificial cooling by suitable control of temperatures,etc.

Upon the application of heat to the outer sheets, such heat will betransmitted, by conduction, toward the center layers of the assembly,such action being accelerated as the bonding material of the outerlayers successively melts, whereby the conduction between the adjacentelements of the structure is increased. Consequently, the bondingmaterial will melt successively from the outer sheets inwardly towardthe center, such melting action taking place uniformly throughout anyplane parallel to the respective layers, whereby the joints in a singleplane will reach bonding temperature simultaneously, and as such actiontakes place, thc pressure applied will be uniformly maintained toprevent any distortion of the structure, and, co-acting with theimproved conductivity of the fused. bonds, speeds up the subsequentmelting of the next layer. Following complete melting of the bondingmaterial, heating is. discontinued and rapid cooling of the assembly isinitiated, as for example, in the structure illustrated, by thecirculation of a cooling medium through the passages in the upper andlower plates. Such cooling will take place in a manner similar to theheating of the assembly, hereby the cooling action will be inwardly fromthe surfaces adjacent the plates, by conduction, and takes placeuniformly throughout any horizontal plane. Thus, as in the case ofheating, distortion and internal stresses are completely eliminated.

Use of the present invention has resulted in the production of heatexchangers, of the type referred to, not only having qualities muchsuperior to those constructed by previous methods, but has enabled theproduction of such exchangers in large sizes and of' great weight,heretofore considered impractical to manufacture. Likewise,

such method has reduced the fabrication time and the amount of heatrequired, both of which highly contribute to the considerable reductionin manufacturing costs achieved it will be noted from the abovedisclosure that the use of the present invention enables the productionof larger and stronger units, with the maintenance of uniform highquality, at a reduction in the cost of manufacture, and the utilizationof production line techniques and procedures.

Having thus described my invention, it is obvious that variousimmaterial modifications may be made in the same without departing fromthe spirit of my invention, hence l do not wish to be understood aslimiting myself to the exact form, order, and number of operationsherein shown and described, or uses mentioned.

What I claim as new and desire to secure by Letters Patent is:

l. The method of fabricating a metal heat exchanger comprising anassembly of a series of stacked slabs of finned passes, each adapted toconduct a iiuid in heat exchange relationship, which comprises thesteps: positioning said assembly of the series of stacked slabs of nnedpasses together with metallic bonding material between the faces to bebonded within an insulated and enclosed chamber which has walls and endswhich substantially contact the walls and ends of the assembly and whichis adapted to be heated within a predetermined temperature range,applying pressure within the range of pressure from substantially fiveto eight pounds per square inch on the top surface of the heat exchangerto uniformly compress a pair of opposite sides of the heat exchangerassembly in a direction substantially parallel to the transversedimension of the exchanger subject to the greatest dimensional changeduring the bonding operation, applying heat to the surface to whichpressure is applied to successively melt, by self-conduction through themetal of the exchanger, the metallic bonding material associated withsuccessive portions of the exchanger lying in planes extendingsubstantially perpendicular to the direction of the pressure applied tothe exchanger assembly to bonding temperature without excessiveannealing of the heat exchanger parts, and maintaining the assemblyimmobile within the insulated chamber, other than the compacting of thestructure resulting from melting of the bonding material, throughout thebonding operation.

2. The method of fabricating a metal heat exchanger comprising anassembly of a series of stacked slabs of finned passes, each adapted toconduct a iiuid in heat exchange relationship, which comprises thesteps: positioning said assembly of the series of stacked slabs offinned passes together with metallic bonding material between the facesto be bonded within an insulated and enclosed chamber which has wallsand ends which substantially contact the walls and ends of the assemblyand which is adapted to be heated and cooled within a predeterminedtemperature range, applying uniform pressure to a pair of opposite sidesof the exchanger assembly in a direction substantially parallel to thetransverse dimension of the exehanffer subject to the greatestdimensional change during the bonding operation to prevent distortion ofthe heat exchanger, applying heat to a surface to which pressure isapplied to successively melt, by self-conduction through the metal ofthe exchanger, said metallic bonding material associated with successiveportions of the exchanger lying in planes extending substantiallyperpendicular to the direction of the pressure applied to the assemblyand following complete melting of the bonding material to bondingtemperature without xcessive annealing of the heat exchanger parts,cooling said exchanger by conduction within the insulated chamber,whereby portions of the bonding material will successively solidify inplanes extending substantially perpendicular to the direction of thepressure applied to the exchanger assembly.

3. The method of fabricating a metal heat exchanger comprising anassembly of a series of stacked slabs of finned passes, each adapted toconduct a fiuid in heat exchange relationship, which comprises thesteps: positioning said assembly of the series of stacked slabs offinned passes together with metallic bonding material between the facesto be bonded within an insulated and enclosed chamber which has wallsand ends which substantially contact the walls and ends of the assemblyand which is adapted to be heated and cooled within a predeterminedtemperature range, applying uniform pressure to a pair of opposite sidesof the exchanger assembly in a direction substantially parallel to thetransverse dimension of the exchanger subject to the greatestdimensional change during the bonding operation to prevent distortion ofthe heat exchanger, applying heat to a surface to which pressure isapplied to successively melt, by selfconduction through the metal of theexchanger, the metallic bonding material associated with successiveportions of the exchanger to bonding temperature without excessiveannealing of the heat exchanger parts lying in planes extendingsubstantially perpendicular to the direction of the pressure applied tothe assembly, cooling said exchanger by conduction following completemelting of the bonding material, whereby portions of the bondingmaterial will successively solidify in planes extending substantiallyperpendicular to the direction of the pressure applied to the exchangerassembly, and maintaining the assembly immobile within the insulatedchamber, other than the compacting of the structure resulting frommelting of the bonding material, throughout the bonding operation. l

4. The method of fabricating a metal heat exchanger comprising anassembly of a series of stacked slabs of finned passes, each adapted toconduct a uid in heat exchange relationship, which comprises the steps:positioning said assembly of the series of stacked slabs of finnedpasses together with metallic bonding material between the faces to bebonded within an insulated and enclosed chamber which has walls and endswhich substantially contact the walls and ends of the assembly and whichis adapted to be heated and cooled within a predetermined temperaturerange, applying uniform pressure to a pair of opposite sides of theexchanger assembly in a direction substantially parallel to thetransverse dimension of the exchanger subject to the greatestdimensional change during the bonding operation to prevent distortion ofthe heat exchanger, applying heat to the surfaces to which pressure isapplied to successively melt, by selfconduction through the metal of theexchanger, the metallic bonding material associated with successiveportions of the exchanger to bonding temperature without excessiveannealing of the heat exchanger parts lying in planes extendingsubstantially perpendicular to the direction of the pressure applied tothe assembly and following complete melting of the bonding material,cooling the surface of said exchanger to which the pressure is appliedby conduction within the insulated chamber, whereby portions of thebonding material will successively solidify in a direction toward thecenter of the assembly in planes extending substantially perpendicularto the direction of the pressure applied to the assembly.

5. The method of fabricating a metal heat exchanger comprising anassembly of a series of stacked slabs of finned passes, each adapted toconduct a uid in heat exchange relationship, which comprises the steps:positioning said assembly of the series of stacked slabs of nned passestogether with metallic bonding material between the faces to be bondedwithin an insulated and enclosed chamber which has walls and ends whichsubstantially contact the walls and ends of the assembly and which isadapted to be heated and cooled within a predetermined temperaturerange, applying uniform pressure to a pair of opposite sides of theexchanger assembly in a direction substantially parallel to thetransverse dimension of the exchanger subject to the greatest unitdimensional change during the bonding operation to prevent distortion ofthe heat exchanger, applying heat to the surfaces to which pressure isapplied to successively melt, by self-conduction through the metal ofthe exchanger, said metallic bonding material associated with successiveportions of the exchanger to bonding temperature without excessiveannealing of the heat exchanger parts lying in planes extendingsubstantially perpendicular to the direction of the pressure applied tothe assembly, cooling the surface of said exchanger to which thepressure is applied by conduction following complete melting of thebonding material within the insulated chamber, whereby portions of thebonding material will successively solidify n a direction toward thecenter of the assembly in planes extending substantially perpendicularto the direction of the pressure applied to the assembly, andmaintaining the assembly immobile, other than the compacting of thestructure re-

